A direct current resistance annealing furnace adapted to be arranged in-line with a drawing machine normally comprises at least two, and in particular three, electric axes, provided with respective pulleys and motorized to feed the metal wire, a plurality of idle or motorized transmission rolls and a motorized outlet pull ring. The transmission rolls and the outlet pull ring are arranged so as to define a given path for the wire, which starts about a first electric axis, turns about the other two electric axes and the transmission rolls and ends about the outlet pull ring.
The annealing furnace comprises an electric apparatus for generating a direct current voltage which is applied between the second electric axis and the other two electric axes, i.e. the positive potential of the electric voltage is applied to the second electric axis and the negative potential of the electric voltage is applied to both the first and the third electric axis. The annealing process occurs by Joule effect due to the current passage in the first wire lengths between the second electric axis and the other two (first and third) electric axes.
The path of the wire is divided into a first pre-heating stretch, which goes from the first electric axis to the second electric axis, a real annealing stretch, which goes from the second electric axis to the third electric axis, and a cooling stretch, which goes from the third electric axis to the outlet pull ring. The pre-heating stretch is longer than the annealing stretch so that the temperature of the wire in the pre-heating stretch is lower than in the annular stretch.
The electric voltage applied between the annealing axes and the corresponding electric current which circulates in the wire are commonly known as “annealing voltage” and “annealing current”, and in general depend on the length of the pre-heating and annealing stretches, on the feeding speed of the wire along the path and on the section of the wire. In particular, it is known to represent the dependence between annealing voltage and feeding speed of the wire by using a so-called annealing curve. According to the annealing curve, the required annealing voltage increases as the feeding speed increases. Furthermore, the annealing current, in general, increases as the cross section of the wire increases. Over given wire section values, the maximum wire speed value is determined by various factors, such as, for example, the cooling capacity of the cooling stretch. It derives that the speed may be high for small cross sections of the wire, to which low annealing currents correspond, and thus the annealing voltage must be high. On the other hand, the speed must be lower for large cross sections, to which high annealing current correspond, and thus the annealing voltage must be lower.
The electric apparatus comprises a three-phase transformer, in which the primary circuit is supplied by the three-phase network, e.g. the 400 V and 50 Hz three-phase network, and a controlled rectifier circuit, which is coupled to the secondary circuit of the transformer to supply the annealing voltage. In order to reach the required annealing temperatures (a few hundreds of degrees Celsius), the transformer is sized to supply an alternating current voltage to the secondary circuit having an amplitude in the order of size of the maximum annealing voltage to be obtained and a maximum annealing current which depends on the overall features of the annealing furnace (wire path length and wire feeding speed) and on the cross section of the wire. For example, the transformer is sized to supply an alternating current voltage of approximately 70 V for a power of approximately 1000 kVA.
The rectifier typically consists of a thyristor bridge (SCR). The modulation of the annealing voltage is obtained by varying the firing angle of the thyristors. In other words, the voltage reduces, starting from the maximum value, with the reduction of the firing angle of the thyristors. However, the firing angle decreases the power factor of the apparatus, i.e. increases the reactive power which is exchanged by the apparatus with the electric network. A high reactive power results in a power engagement of the electric network which does not result in a creation of active work. Furthermore, the national authorities which control the distribution of electricity on the power network normally apply penalties when the reactive power exceeds a given percentage of the delivered active power.
A further disadvantage of the apparatus described above is the cumbersome size of the transformer, which is in fact oversized for its use because it never supplies the maximum current at the maximum voltage to the secondary circuit.
An electric apparatus which overcomes some of the drawbacks of the apparatuses described above is known. This other apparatus differs from the described one substantially in that it comprises a transformer with a plurality of tap points on the primary circuit. The tap point of the primary circuit which allows to maximize the firing angle of the thyristors of the rectifier and thus to minimize reactive power is selected according to the section of the wire to be annealed. However, the transformer with multiple tap point primary circuit is also oversized, and in all cases more complicated and costly than a transformer with a simple primary circuit. Furthermore, it is economically inconvenient to construct large-sized transformers (e.g. 70 V for 1000 kVA on the secondary circuit) with more than four tap points on the primary circuit.
A known architecture alternative to the use of a transformer with multiple tap point primary circuit comprises a simple primary circuit transformer and an AC/AC inverter coupled to the primary circuit of the transformer to adjust the power voltage of the primary circuit to a higher number of levels and thus correspondingly adjust the voltage supplied by the secondary circuit. This solution allows to reduce the reactive power further, but the drawbacks related to large sized transformer remain.